1. Field of the Invention
The present invention relates to an image processing apparatus and an image processing method for synthesizing plural sheets of images.
2. Description of the Related Art
To photograph a night scene with an image pickup apparatus such as a digital camera that is hand-held, photographing is performed with high sensitivity to suppress hand-shake, however, this results in increased noise. It is therefore known to align plural sheets of images photographed with high sensitivity, arithmetically average pixel values of the images on a per pixel basis, and synthesize the averaged pixel values, thereby obtaining a less noisy image while suppressing hand-shake.
An image pickup apparatus is also known, in which a single sheet of strobe-illuminated image photographed while irradiating light onto a photographic subject from a light emission unit (strobe unit) is synthesized with plural sheets of non-strobe-illuminated images photographed without using the light emission unit (see, for example, Japanese Laid-open Publication No. 2007-124292).
In this image pickup apparatus, when photographing is performed while causing the strobe unit to emit light, sensitivity is set to be low. When photographing is performed while not causing the strobe unit to emit light, sensitivity is set to be high. Next, ratios for synthesis of the strobe-illuminated image and the non-strobe-illuminated image are decided on a per pixel basis, and these images are synthesized based on the decided synthesis ratios. For example, a brightness value of each pixel of the strobe-illuminated image is compared in magnitude with a brightness value of a corresponding pixel of the non-strobe-illuminated image, and these images are synthesized by using pixels, whichever are greater in brightness value (i.e., by setting synthesis ratios of 100% for pixels which are greater in brightness value).
However, in a case, for example, that there is a shadow area in a human subject area illuminated with illumination light, the strobe-illuminated image is not used for the human subject area in some cases. In that case, the synthesized image becomes unnatural.